Add draft style guide

[marius311]

Adding here for comments, before going through the code and actually implementing this.

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[xzackli]

My other concern would be guidance on unicode usage.

• Unicode right now doesn't have a complete set of subscripts and superscripts, and stuff like \prime\prime and \pprime worry me a bit.
• Even usage of \bb, \bf etc can cause problems, due to bugs in other stuff (tab completion of surrogate pairs causes error, possible regression of #2255 jupyterlab/jupyterlab#9524)
• certain monospace fonts don't have great math support

[marius311]

My other concern would be guidance on unicode usage.

Agree that should definitely be added to the guide.

Unicode right now doesn't have a complete set of subscripts and superscripts, and stuff like \prime\prime and \pprime worry me a bit.

Yea, for this reason I'd say discourage broad use. So I like that we're doing Ω_r, Ω_b, Ω_m etc... I would actually consider getting rid of the _ though, I don't think its any less clear without.

Even usage of \bb, \bf etc can cause problems, due to bugs in other stuff (jupyterlab/jupyterlab#9524)

Yea, I'm not married to 𝕡 type stuff although I do like it alot. Would be good to standardize the naming a bit, eg 𝕡, bg, ih, etc... should be one style.

certain monospace fonts don't have great math support

I'd say we recommend Julia Mono and in the guide say that if it doesn't look good in Julia Mono don't use it, but if it does fair game.